(1) Field of the Invention
This invention generally relates to a baffle system for mitigating thruster wake deficit. More particularly, the invention relates to a baffle system for mitigating thruster wake deficit having extendable baffles which direct and thereby control boundary layer flow along an exterior surface of a vessel, and thereby improve maneuvering of the vessel.
(2) Description of the Prior Art
The current art for thrusters includes installation of a rotating propeller in a tunnel formed transversely through the vehicle. The rotating propeller creates a pressure differential across the propeller blades and drives a jet of water through the tunnel and out one side. The integrated pressure force on the propeller blades is transferred to the vehicle via a rotor hub as a force acting in a direction opposite to that of the jet flow. This effect is used to maneuver the vehicle. Even further, it is known in the art to design thrusters which are reversible thereby enabling maneuvering of the vehicle in either port or starboard directions.
Early efforts to measure the effects of forward vehicle velocity on tunnel thruster performance showed that as the forward velocity of the vehicle was increased to a speed on the order of 3 knots, the effective side force (force perpendicular to the longitudinal axis of the vehicle) from the tunnel thruster decreased to as low as 10 percent of the side force measured at zero forward vehicle velocity. Thus, a problem exists in the art whereby tunnel thrusters quickly lose their maneuvering effectiveness as forward vehicle velocity increases. Experiments conducted to understand this phenomenon indicated that it is not the forward velocity per se which significantly alters the force acting on the vehicle through the propeller hubs. Instead, the inventors of the present invention have discovered that the thruster jet acts as an obstruction to the boundary layer flow over the vehicle hull. More specifically, a wake deficit is created in the boundary layer downstream of the thruster jets. The resulting wake-induced pressure deficit on the vehicle surface generates an integrated suction force on the hull that counteracts the force on the blades. Conversely, on the suction side of the tunnel there is a high pressure stagnation region aft of the tunnel due to the vehicle boundary layer being sucked off by the thruster. The integrated force in this high pressure region also counteracts the force on the thruster blades.
Tunnel thrusters are typically reversible which means that the blades can be rotated clockwise or counterclockwise to produce a jet in either direction through the tunnel and thus provide force in either direction to maneuver the vehicle. Thus, any device that is deployed to mitigate the effects of forward velocity must also be reversible.
The following patents listed below, for example, disclose various types of tunnel thrusters for water vehicles, but do not disclose a baffle system for a tunnel thruster which will deflect water at the flow surface of the vehicle to assist in counteracting or minimizing the properties exhibited by the boundary flow layer due to the thruster jet, particularly at predetermined vehicle speeds. These patents are:
U.S. Pat. No. 3,008,443 to Blickle; PA1 U.S. Pat. No. 3,408,974 to Pehrsson; PA1 U.S. Pat. No. 3,710,748 to Baer et al.; PA1 U.S. Pat. No. 3,797,447 to Stubblefield; PA1 U.S. Pat. No. 3,830,184 to Krautkremer; PA1 U.S. Pat. No. 4,018,181 to Brix; PA1 U.S. Pat. No. 4,455,960 to Aker.
Specifically, the patent to Blickle discloses a device for covering transverse passages in ships. More specifically, the device is equipped with a propulsion elements arranged in the transverse passage. It is an object of the invention to provide a device for covering transverse passages in ships with a propulsion element in the passages, which will effectively protect the propulsion element against foreign floating bodies, even if the means for closing the transverse passage occupy their fully opened position. This patent makes no claims with reference to the decrease in effective thrust with forward speed, makes no reference to pressure differential across the hull, and does not disclose a system of baffles to deflect a portion of the flow to reduce the pressure differential across the hull and improve the effective force generated by the thruster.
The patent to Pehrsson discloses a ship steering system which includes tunnels extending transversely through a ship's hull at the bow or stern or both in which is mounted a reversing or reversible pitch propeller in order to pump water selectively through the tunnel to exert a steering force on the hull and including vanes or screens which can be extended outwardly from and withdrawn into the hull located behind the ends of the tunnel or tunnels in the direction of movement of the ship in order to exert a turning force on the hull and also to direct water selectively in the tunnel during the forward or rearward movement of the ship to enable control of the steering of the ship either at low or high speeds. Thus, Pehrsson fails to disclose a vane extending outward on the suction side of the thruster, or a system of baffles to deflect a portion of the discharge flow along the hull to reduce the pressure differential across the hull and improve the effective moment generated by the thruster.
Baer et al. discloses a steering device in which a longitudinal flow passage opens at the bow of a ship and has impeller means therein with first and second discharge flow passages branching from the longitudinal passage behind the impeller and opening on both sides of the hull. Controllable valve means in the discharge flow passages control the flow of water therethrough with the water being discharged from openings whose rear edges project outwardly of the hull surface a distance about one fourth of the width of the discharge opening. Accordingly, Baer et al. are directed to a thruster system with a longitudinal passage that opens at the bow, including controllable flap valves for directing the flow to one side of the ship or the other. An alternative is disclosed in which the controllable flap is positioned at the discharge opening. This does not refer to the decrease in effective thrust with forward speed, to pressure differential across the hull, or a system of baffles to deflect a portion of the flow to reduce the pressure differential across the hull and improve the effective force generated by the thruster. In fact the controllable flap as configured in FIG. 2 thereof would act to increase the pressure deficit downstream of the thruster jet and thus decrease the effective force of the thruster with forward velocity.
Stubblefield discloses an inboard propulsion system for a boat utilizing a water jet propulsion characterized by a pair of spaced nozzles each provided with individually controlled deflecting hoods to enable providing both a reverse thrust for backing the boat and to selectively reverse the water jet of a single nozzle to provide a turning force for steering the boat. Preferably, each of the nozzles is provided with a servo system which varies the effective opening of the nozzle in response to changes in the pressure differential between the intake pressure to the main impeller unit and the discharge pressure of the impeller unit to attempt to maintain a constant quantity flow from the nozzles independent or regardless of any variations in the intake pressure of the impeller unit. Thus, the reference is directed to a primary propulsion system for a boat located on the stern of the boat and to controlling deflector hoods backing or steering the boat. There is no reference to secondary tunnel thrusters extending laterally through the hull for maneuvering the boat at low speeds and no reference to a decrease in effective thrust with forward speed. No reference is made to pressure differential across the hull or a system of baffles to deflect a portion of the flow to reduce the pressure differential across the hull and improve the effective force generated by the thruster as in the present invention.
Krautkremer discloses an attachable or a detachable unit providing a lateral thrust rudder for ships. This reference includes a unitary mechanism constituting a tunnel, a propeller within the tunnel and driving means for the same which can be bodily mounted into or detached from a ship. When the mechanism is in an operating position, it is normally mounted at the bow of the ship and functions to apply a lateral thrust in one direction or the other to the bow. Preferably, the unit is so mounted that the driving mechanism projects into the interior of the ship for easy access thereto. Thus, the reference includes a thruster system that is designed and built separately from a ship hull and can be added or not as a separate component which is not an integral part of the ship. There is no reference to the decrease in effective thrust with forward speed or to pressure differential across the hull. There is no system of baffles disclosed to deflect a portion of the flow to reduce the pressure differential across the hull and improve the effective force generated by the thruster as occurs in the present invention.
Brix is directed to a lateral thrust control unit for watercraft having a pair of tunnels which are directed transversely to the longitudinal axis thereof. Each of the tunnels extend from one side of the watercraft the oppositely positioned side of the watercraft and have at least one drivable propeller therein. At least one pressure compensating channel is provided near the tunnels and connects at least one of the zones of the differing pressure fields created on the sidewalls of the watercraft as it moves simultaneously longitudinally and laterally to the pressure field of different potential to equalize the pressure differential there between and to reduce the resistance to the lateral movement. The pressure-compensating channels do not have any propulsion devices therein. Thus, Brix merely provides a pressure compensating channel to equalize the pressure differential and makes no reference to the decrease in effective thrust with forward speed. There is also no reference to a system of baffles to deflect a portion of the flow to reduce the pressure differential across the hull and improve the effective force generated by the thruster.
Aker is directed to a boat thruster system including a pump for drawing water through an inlet in the boat hull and for discharging water through first and second pipes connected to outlets located on either side of the hull. A valve is installed in each of the pipes to control the flow of water therethrough. The valves may be controlled by either an open or closed loop control system configured so as to prevent both outlet pipes from being closed at the same time during system operation. Each valve is preferably comprised of multiple vanes each of which is mounted for rotation about an off center axis such that in the event of a valve control system failure, the water flow will cause the valve to open rather than close thereby preventing undesirable high pressure buildup in the system. Aker does not refer to a decrease in effective thrust with forward speed or to pressure differential across the hull. Also, there is no reference to a system of baffles to deflect a portion of the flow to reduce the pressure differential across the hull and improve the effective force generated by the thruster as occurs in the present invention.
It should be understood that the present invention would in fact enhance the functionality of the above patents by diverting the flow of water at the surface of and thereby over the vessel hull, thus mitigating a surface pressure differential across the vehicle downstream of the thruster jet and thus eliminate the force which counteracts the force on the thruster blades. Further, the present invention provides the advantage that the effective control force produced by the tunnel thruster will not decrease significantly with forward speed on the vehicle. Thus, the performance of the thruster in maneuvering the vehicle with forward speed will be improved by this invention. The baffle feature directs the flow into and out of the tunnel thruster in a manner not previously recognized in the art.